I can't speak

Last night I went to a Mexican restaurant before going to the movies with a friend. My friend immediately talks in Spanish whenever she knows what to say. ("Sí", "gracias", "Buenas noches"). The waitress asks her if she speaks Spanish, and she replies, "Un poquito". She then asks me if I speak Spanish, and I say "Un poquito más que ella." Then she asks me in Spanish what I want to order, and my mind goes totally blank. I say "¿Cómo?", then she answers me in English. Why do I always get so nervous when I have to speak in Spanish? I knew she was asking me what I wanted to order. All I had to say was "un burrito con vegetales" or "un burrito con verduras". I have conversations with tutors and everything. What is the purpose of knowing that you can say "papel higiénico" if you need to ask for toilet paper (my friend needed this phrase) if you're not going to say it to someone?